Riverine environments like the Hudson present certain challenges to photosynthetic organisms (Cloern, 1987; Alpine and Cloern, 1988). The water column of the tidal-freshwater Hudson is well mixed and turbid. The suspended particles absorb light; the full water-column mixing ensures that organisms suspended in the water are rarely in the surface where light is highest (Fig. 9.1). At the average depth (9 m) and light penetration for the Hudson, the average phytoplankton spends from 18 to 22 hours in light too dim for net positive photosynthesis to proceed (Cole et al., 1992). The situation differs in the saline parts of the estuary and harbor where the water column is stratified, at least some of the time, leading to shallower mixing depths (Swaney, Howarth, and Butler, 1999), and concomitantly higher rates of primary production (Chapter 10). Nevertheless, low light is still a major growth-limiting factor in the lower estuary as well (Garside et al., 1976; Malone, 1977).
Attached to the bottom, macrophytes and peri-phyton are restricted to extremely shallow water (< 1 m) due to low light. On the other hand, these attached plants are less affected by advective loss than are phytoplankton. While the net freshwater flowoftheHudsonisnotveryrapid, photosynthesis of suspended organisms needs to exceed the advec-tive losses if biomass is to increase at a given site. During the growing season a typical residence time for water in the tidal-freshwater river is about 30 to 50 d, or 2 to 3 percent per day. To simply sustain biomass at a given location then, net growth, after respiratory and predatory losses are subtracted, must be at least this large.
In many aquatic environments the supply of essential nutrients for plant growth, typically phosphorus (P) or nitrogen (N), and some trace metals (iron, selenium e, etc.) limits the net growth of phytoplankton. In some rivers and most estuaries, since trace metals are generally high, N and P are the likely limiting nutrients (Howarth, 1988; Fisher et al., 1992). Such is not the case in the Hudson. In the tidal-freshwater river, for example, NH4 depletes from winter values near 10 |M to fairly low values in mid summer (~2 |M). NO3 varies seasonally between wintertime highs of near 50 |M and summertime "lows" above 30 |M. PO4 values are lowest in spring (0.4 to 0.5 |M) and increase in late summer (at the peak of phytoplankton biomass) to as much as 0.8 to 1 |M (Fig. 9.2; Lampman, Caraco, and Cole, 1999). If either PO4 or NO3 were limiting one would expect a negative correlation with phytoplankton biomass, which is not seen at all in the Hudson.
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